Is she or isn’t she? This is the question, and the tantalizing draw, of My Cousin Rachel, a new adaptation of a Daphne du Maurier novel. The story is the sort of genteel-gothic potboiler that du Maurier mastered in Rebecca, and like that novel it features a woman arriving at a mansion where mystery awaits. But this forceful new arrival is far from the meek, unnamed bride of Rebecca—she herself is the source of the mystery.